When scanning original printed music, what is the best page size to choose?

I set up my scanner (an old Ricoh Aficio 1032 A3 office machine) to scan about 3mm less than the page size, so as to minimise black edges. When converting to pdf, I can increase the margins to create an identical page size to the original.

However, when I want to print out parts downloaded from IMSLP, most music is published with a page size usually well in excess of A4. In order to maximise the stave size when printed as an A4 booklet, I trim the margins first.

To summarise- is it better to be true to the original, or anticipate the eventual printing on A4?

Most scans of before-1900 scores made by libraries - left column of http://imslp.org/wiki/IMSLP:Other_music_score_websites - encompass their whole original size - intermediate between A3 and A4 - and sometimes add ugly margins. I have scanned a lot of such scores. The area of [systems + page numbers + plate numbers] in the original score almost always fits into an A4 area because of margins bigger than 1 inch. I imagine such margins were technically necessary in the copper-plate printing times. So I strongly recommend your adjusting the margins to about .3 or .4 inches if your scan is intended for playing.